In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. The inventors expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art.
Cutting tools are usually provided with a coating deposited by either physical vapour deposition (PVD) or chemical vapour deposition (CVD) to increase the tool life. Common PVD coatings for cutting tools are nitrides of Ti, Al, Cr and Si. Nitrides containing silicon are known to have a high hardness and very good wear resistance. (Ti,Al,Cr,Si)N PVD coatings having a high hardness are also known in the art.
EP 1939328 A1 describes a cutting tool comprising a substrate and a coating. The coating comprises an aperiodic, multilayered structure of alternating individual metal nitride layers X and Y of different composition where the average composition of the coating is (Ti(1-a-b-c)AlaCrbSic)N where 0<a<0.5, 0<b<0.15, 0.01<c<0.17, and a+b+c<1.
US 2006/0222893 A1 discloses a multilayer coating comprising repeated layer stacks comprising at least one, 50-150 nm, (Al,Cr)N layer and/or at least one, 75-200 nm, (Ti,Si)N layer and at least one layer stack of (Al,Cr,Ti,Si)N+(Ti,Si)N+(Al,Cr,Ti,Si)N+(Al,Cr)N. The mixed (Al,Cr,Ti,Si)N layers have a multilayered structure and are achieved by running all targets simultaneously. The (Al,Cr,Ti,Si)N layer has a thickness of 20±10 nm.
EP 1 726 686 A1 discloses a hard-coated member comprising a coating comprising a lowermost layer, an intermediate laminate layer and an outermost layer where the lowermost and outermost layers are homogenous layers. The intermediate laminate layer comprises alternating layers A and B, where A and B are nitrides, borides, carbides and oxides of the metal components AlwCrxTiySiz.
Although such hard coatings have relatively good wear resistance, they can also be brittle, which may cause problems, like flaking, when deposited onto substrates which have high toughness and also onto substrates having sharp geometries. If the coating also has high residual compressive stresses the flaking can be even worse, especially around cutting edges, corners on drills, etc.